Question: $ \left(\dfrac{25}{81}\right)^{-\frac{1}{2}}$
$= \left(\dfrac{81}{25}\right)^{\frac{1}{2}}$ Figure out what goes in the blank: $\Big(? \Big)^{2}=\dfrac{81}{25}$ Figure out what goes in the blank: $\Big({\dfrac{9}{5}}\Big)^{2}=\dfrac{81}{25}$ So $\left(\dfrac{25}{81}\right)^{-\frac{1}{2}}=\left(\dfrac{81}{25}\right)^{\frac{1}{2}}=\dfrac{9}{5}$